A small unmanned air vehicle (UAV) has been developed. Such a UAV at the present moment is mainly a fixed wing type or a rotary wing type. However, as a future UAV form, development of “flapping type” which imitates birds or insects is initiated. The flapping type UAV has characteristics of both the fixed wing type and the rotary wing type, and is expected from the viewpoint of quietness, low visibility, mimetic characteristics, etc. Such a flapping type UAV is hereinafter referred to as an “ornithopter”.
Patent Literature 1 (Japan Patent No. 4,675,346) discloses an example of the ornithopter. This ornithopter includes a main body, and a pair of wings attached to the main body. One of the pair of wings is attached to a left side portion of the main body, and the other of the pair of wings is attached to a right side portion of the main body. Each wing is a single piece wing. The ornithopter floats and travels in the air by a flapping motion and a torsional motion of the pair of the single piece wings.
In the ornithopter disclosed in Patent Literature 1, a torsional motion of a wing is actively controlled by an ultrasonic motor. In the case of adopting the active control, an extra actuator and an extra wiring are required. Therefore, problems such as weight increase and complication of a mechanism occur. Then, the weight increase and the complication of the mechanism cause problems such as increase of thrust loss and drag. In addition, since the way of actively twisting the wing is different from a mechanism in a natural bird, the mimetic characteristics are deteriorated.